1. Field of the Invention
The present invention pertains to a method of manufacturing MOS transistors, and more particularly, to a method that utilizes a high-tensile thin film in combination with an annealing process to alter a band structure of a channel region in a substrate, thereby improving carrier mobility of the MOS transistors.
2. Description of the Prior Art
The performance of MOS transistors has increased year after year with the diminution of critical dimensions and the advance of large-scale integrated circuits (LSI). However, it has been recently pointed out that the miniaturization attained by a lithographic technology has reached its limit. Therefore, how to improve the carrier mobility so as to increase the speed performance of MOS transistors has become a major topic for study in the semiconductor field. For the known arts, attempts have been made to use a strained silicon layer, which has been grown epitaxially on a silicon substrate with a silicon germanium (SiGe) layer disposed therebetween. In this type of MOS transistor, a biaxial tensile strain occurs in the epitaxy silicon layer due to the silicon germanium which has a larger lattice constant than silicon, and, as a result, the band structure alters, and the carrier mobility increases. This enhances the speed performance of the MOS transistors.
However, the above technique still suffers the following disadvantages. First, the silicon germanium is deposited across the substrate, making it harder to optimize NMOS and PMOS transistors separately. Second, the silicon germanium layer has poor thermal conductivity. Another concern is that some dopants diffuse more rapidly through the SiGe layer, resulting in a sub-optimum diffusion profile in the source/drain region.